Will This Fit? — Context Window Calculator

Paste a document and see which models it fits inside, how much of each window it fills, and what is left for the reply.

This calculator measures a document in tokens and checks it against the context window of every major model, showing how much of each window it fills, whether it still fits once you reserve space for the reply, and what a single call would cost. Message overhead is included, because role markers and separators make a long conversation measurably larger than the text alone.

Runs in your browser

Largest window here: 1M tokens — roughly 750,000 words
Shared budget: Input and output come out of the same window on most models
Privacy: Runs entirely in your browser — nothing is uploaded
Cost: Free, unlimited, no sign-up

Frequently asked questions

Does the reply come out of the same window?

Yes on almost every current model — max_tokens is a cap on the reply, but the reply still has to fit alongside the prompt. That is why this tool asks you to reserve space: a prompt that fills 99% of the window leaves no room for an answer.

How many words is a million tokens?

Roughly 750,000 words of English prose, or about 2,500 pages. Code and JSON are far denser — the same window might hold only 400,000 words of source.

How to use the context window calculator

1Enter or paste your document or prompt.
2Set the reserve for the reply.
3Set the messages in the conversation.
4Choose the count using.
5Press Run, then download the result when it is ready.

Sources & specifications

Embed this tool

Put the working context window calculator on your own site. It runs in your visitors' browsers exactly as it does here — free, no account, nothing uploaded.

LLM Token CounterCount tokens with the real BPE vocabulary, see every token coloured in place, and compare the count across models.LLM Model ComparisonEvery current model side by side: price per million tokens, context window, max output, modalities and cache rates.Prompt Template GeneratorWrite a prompt once with placeholders, paste rows of data, and get every filled prompt back with its token count and cost.Prompt Token OptimizerStrip the padding out of a prompt — dead whitespace, boilerplate politeness, redundant framing — and see the tokens and money saved.Words to Tokens ConverterConvert between words, characters, pages and tokens using measured ratios for prose, code, JSON and CJK.Prompt Injection SanitizerStrip invisible carriers, neutralise instruction-like markup and fence untrusted content before you paste it into a prompt.

Last updated 1 Aug 2026

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